In optical storage media such as optical disks in general and DVDs in particular, data is stored along tracks formed in the bulk of the optical disk and is read by focusing a laser beam produced by semiconductor diodes on to the tracks, while spinning the disk on its axis. The tracks generally comprise spiral tracks on which data is written and from which the data is read.
Obviously in order to read data correctly it is essential that the reading head can locate and follow a desired track. In practice this leads to two different kinds of tracking problem: skipping from one track to another and faithfully following a single track. The first problem must be addressed, for example, when data is read from or written to a first track and the user selects a different track for reading data therefrom or writing data thereto. The second problem must be addressed when data is being read from or written to a required track, in order to ensure that data is not inadvertently read from or written to an adjacent track. For the purpose of the present discussion, it suffices to observe that these two different tracking problems require different solutions and the present invention is concerned only with the second of the two problems.
In a CD data is written on a single surface only, while in a DVD data is written on two surfaces. However, the manner in which data is read from both media is essentially the same in that a single laser beam is focused to a spot on the writing surface and tracks the data thereon. In both cases, the data is recorded in tracks which are typically spiral. In the case of CDs, where data is recorded on a single layer only, the laser beam is focused to the refractive plane. Against this, in DVDs where data is recorded on two layers, the read/write head is adapted to be raised or lowered in order that the laser beam be focused on the correct layer. However, once this is done, planar tracking is performed in a similar manner to CDs.
The manner in which the reading head tracks a destination track is based on focusing the reading spot on to the track and measuring the intensity of a reflected spot by position sensitive detectors. This allows calculation of the position of the reading spot and subsequent adjustment of the reading head's location based on the measured error.
US20010040844A1 published Nov. 15, 2001 (Sato et al) entitled “Tracking servo apparatus of optical information recording and reproducing apparatus” discloses a tracking servo apparatus using this technique. Thus, reflection light obtained when a laser beam is irradiated onto a recording surface of an optical disc is photoelectrically converted, thereby obtaining a photoelectric conversion signal. A tracking error signal showing an amount of deviation of an irradiating position of the laser beam for a track in a disc radial direction on the recording surface is generated by the photoelectric conversion signal. A spherical aberration occurring in an optical system is detected, a level of the tracking error signal is corrected on the basis of the detection result, and the irradiating position of the laser beam is moved in the disc radial direction in accordance with the level-corrected tracking error signal.
Likewise, U.S. Pat. No. 6,233,210 published May 15, 2001 (Schell; David L.) entitled “Optical drive error tracking method and apparatus” discloses a method and apparatus for obtaining a tracking error signal for an optical disk player which is general across the various data formats found in CD audio disks and DVDs. A photodetector having at least four active areas is used to sense the reflected laser beam. A differential amplitude tracking error signal is generated by comparing the signal strength in the different active areas.
These references are typical of known solutions for maintaining the read/write head in communication with a desired track using a photodetector having multiple sections that serves as a position-sensitive detector for detecting a component of the read/write laser beam reflected from the surface of the optical disk.
For both CDs and DVDs, axial compensation translates to a focusing adjustment of the read/write beam.